Various aspects of the present invention relate in general to contact photolithography, and in particular, to techniques for making high resolution photomasks for use with photolithographic processes.
In conventional photomask manufacturing, a rigid transparent substrate such as glass or quartz is coated with a thin film layer of chromium. To create a pattern in the photomask, a layer of photoresist material is deposited over the film layer of chromium. An exposure operation is then performed, whereby the photoresist material on the substrate is selectively exposed to light, e.g., from an ultraviolet (UV) source, according to the desired mask pattern. The photoresist material is photosensitive to the light utilized in the exposure operation, thereby altering the chemical resistance of the exposed regions of the photoresist material to a corresponding developer.
The mask pattern is subsequently developed in the photoresist material by using a liquid developer to remove portions of the photoresist material from the substrate, thus exposing the underlying film layer of chromium through the photoresist in a manner corresponding to the mask pattern. The exposed layer of chromium is then etched away from the transparent substrate using a wet chemical etchant. Finally, the photoresist remaining on the film of chromium is removed using solvents, resulting in a final mask product defined by areas of the mask that are opaque to light emitted by an ultra violet (UV) lamp utilized in a subsequent photolithographic process, and areas of the mask that are transparent to light from the UV lamp utilized in the subsequent photolithographic process.
As an alternative to the use of glass or quartz, the photomask substrate may comprise a polymer film, e.g., an acetate film, a Mylar film or other similar material. In a conventional film photomask process, a pattern is developed in an emulsion applied to the surface of the film using a suitable chemical developer. Additional processing may further be required to prepare the mask for use in its intended application. However, with such conventional film mask processing techniques, the resolution is limited. For example, current film photomasks used in photolithography processes are unable to resolve images less than 8-10 microns accurately or repeatedly. Thus the typical feature size is limited, and chemical processing is required to create a pattern over the surface of the film.